Method and apparatus for gas purification

ABSTRACT

This invention comprises an adsorption process for the removal of at least N 2 O from a feed gas stream that also contains nitrogen and possibly CO 2  and water. In the process the feed stream is passed over adsorbents to remove impurities such as CO2 and water, then over an additional adsorbent having a high N 2 O/N 2  separation factor. In a preferred mode the invention is an air prepurification process for the removal of impurities from air prior to cryogenic separation of air. An apparatus for operating the process is also disclosed.

This application claims the benefit of U.S. provisional applications60/342,673, filed Dec. 20, 2001 and Ser. No. 60/384,611, filed May 31,2002 the entire teachings of both are herein incorporated by reference.

FIELD OF THE INVENTION

This invention relates to the removal of N₂O, hydrocarbons, water vaporand CO₂ from gas streams, and more particularly to the removal ofimpurities from air, using adsorptive separation, prior to cryogenicseparation of air.

BACKGROUND OF THE INVENTION

Cryogenic separation of air requires a pre-purification step to removecontaminants such as water, CO₂ and hydrocarbons from air. In coldsections of the separation process (such as heat exchangers and LOXsump), water and CO₂ can solidify and block the heat exchangers or othercomponents in the distillation columns. Acetylene and other hydrocarbonsin air present a potential hazard. The high boiling hydrocarbons canaccumulate in the liquid oxygen and create an explosion hazard. Thus,those impurities in air must be removed in an adsorptive clean-upprocess prior to the cryogenic distillation of air.

Nitrous oxide (N₂O) should also be removed from air prior to separation.N₂O is currently present in air at a concentration of about 300-350 ppb,however, this concentration is increasing annually at a rate of about0.3%. Various factors such as emissions from motor vehicles, HNO₃plants, adipic acid and caprolactam plants (both use HNO₃ for oxidationof inorganics) contribute to this growing ambient concentration of N₂O.The presence of greater than 50 ppb of N₂O can be a serious problem forcryogenic air separation units (ASU) because it can form solid depositsin distillation columns. N₂O also decreases the solubility of CO₂ inliquid oxygen, thereby increasing the potential for freezing of CO₂ inthe distillation columns. This can result in degraded performance andcan even cause blockage of heat exchangers.

Air prepurification can be accomplished using pressure swing adsorption(PSA), temperature swing adsorption (TSA) or a combination of both(TSA/PSA) incorporating either a single adsorbent or multipleadsorbents. When more than one adsorbent is used, the adsorbents may beconfigured as discrete layers, as mixtures, composites or combinationsof these. Impurities such as H₂O and CO₂ are commonly removed from airusing two adsorbent layers in a combined TSA/PSA process. Normally, afirst layer of activated alumina is used for water removal and a secondlayer of 13× molecular sieve is used for CO₂ removal. Prior art, such asU.S. Pat. No. 4,711,645, teaches the use of various adsorbents andmethods for removal of CO₂ and water vapor from air.

Centi et al. (Ind. Eng. Chem. Res., vol. 39, pp 131-137, 2000) studiedthe behavior of various ion exchanged forms of ZSM5 Zeolites for removalof relatively high concentrations of N₂O (500 ppm (parts per million) to2000 ppm) from industrial gas streams. ZSM5, being a high Si/Al ratio(2-200) zeolite, has less water affinity than its low Si/Al ratiocounterparts. The best performance for N₂O removal in Centi's study isshown by Ba and Sr exchanged ZSM5. The paper indicates that in thepresence of water, metal exchanged ZSM-5 has better N₂O adsorptionproperties than lower Si/Al ratio zeolites such as X and Y typezeolites.

U.S. Pat. No. 6,106,593 teaches a process, preferably TSA, that uses athree-layer adsorbent bed for successive removal of water, CO₂ and N₂O,wherein the preferred adsorbent is binderless CaX. Other adsorbents suchas CaX (with binder), BaX and Na-mordenite are also recommended for thethird layer. According to the patent, the criteria for selecting anadsorbent for N₂O removal is a Henry's law selectivity for N₂O comparedto CO₂ of 0.49 or more at 30° C. and a Henry's law constant for N₂Oadsorption of at least 79 mmol/gm.

European patent application EP 0 862 938 teaches the placement of azeolite adsorbent selected from X-zeolite, Y-zeolite, A-zeolite ormixtures thereof downstream of an alumina adsorbent in a PSA process toremove nitrogen oxides, such as NO, NO₂, N₂O and N₂O₃. European PatentApplication EP 0 995 477 teaches a method of removing at least a portionof N₂O in a gas stream using a type-X zeolite with a Si/Al ratio of1.0-1.5 and containing a mixture of K⁺(<35%), Na⁺(1-99%) and Ca²⁺(1-99%)cations in various proportions.

European Patent Application (EP 1 092 465) teaches a TSA process(sequentially removing H₂O, CO₂ and N₂O and optionally hydrocarbonsusing a three-layer configuration of adsorbents. A NaLSX adsorbent ispreferred in the second layer for CO₂ removal. A LSX zeolite(Si/Al=0.9-1.3), preferably CaLSX zeolite, is suggested for N₂O andhydrocarbon removal.

European Patent Application EP 1 064 978 teaches the use of BaX zeoliteto remove propane, ethylene and N₂O in a PSA or TSA process. The BaXzeolite contains at least 30% barium cations.

U.S. Pat. No. 4,156,598 teaches the method of removing N₂O from nitrogentrifluoride by passing the gas through a synthetic zeolite adsorbent,such as sodium or calcium exchanged type X or type A zeolite.

U.S. Pat. No. 4,933,158 teaches a method of removing N₂O and CO₂ fromnitrogen trifluoride by passing the gas through a thermally treatedzeolite selected from the group consisting of analcime, clinoptilolite,mordenite, ferrierite, phillipsite, chabazite, erionite and laumotite.

U.S. Pat. No. 4,507,271 teaches the method of removing N₂O from a gascontaining hydrogen, nitric oxide and nitrous oxide using A, X or Yzeolite.

U.S. Pat. No. 5,587,003 discloses a method for removing substantiallyall of the CO₂ from air using the adsorbent clinoptilolite.

Rege et al. (Chemical Engineering Science, vol. 55, pp 4827-4838, 2000)showed 13× adsorbent to provide better CO₂ removal from air thanclinoptilolite. Rege also showed that Ca-exchanged clinoptilolite tohave low N₂ adsorption.

Catalytic decomposition of the contaminant is another means of removingan undesirable component from a gas mixture. A catalyst/adsorbent can beused much in the same way as described above except that the product ofdecomposition must be either removed as an additional contaminant or bean acceptable component of the gas mixture.

The prior art has typically derived its solution to the problem byseeking adsorbents with high N₂O to CO₂ selectivity. However, given thesimilar electronic structure of N₂O and CO₂, and the nearly 1000-folddifference in gas phase concentration between N₂O and CO₂ in air, thismethodology is difficult to apply. Thus an improved process andapparatus for the removal of N₂O and other impurities from air isrequired.

SUMMARY OF THE INVENTION

In a preferred embodiment of the invention, CO₂ and water are removedfrom the feed air, then an adsorbent having a high N₂O/N₂ separationfactor is used for N₂O removal. Such adsorbent also has a higher Si/Alratio and modest to low N₂O/CO₂ selectivity as compared to the priorart.

In a preferred embodiment, the invention relates to an adsorptionprocess for the removal of N₂O from a gas containing N₂O, nitrogen andother components to produce a product gas, said process comprisingpassing said gas over a bed of one or more adsorbents, wherein at leastone of the adsorbents is selected from the group consisting ofclinoptilolite, chabazite and Li-exchanged zeolite or combinationsthereof.

In a more preferred embodiment, the gas is air and the other componentsinclude water and CO₂.

In one embodiment N₂O is in said gas in an amount of less than 100 ppm.

In one embodiment the water and the CO₂ are adsorbed on an additionaladsorbent prior to the gas passing over the clinoptilolite, chabazite orLi-exchanged zeolite.

In one embodiment the process is an air prepurification process.

In a preferred embodiment, at least 90% of the N₂O in the gas isadsorbed.

In one embodiment the Li-exchanged zeolite is LiX.

The invention also comprises a process for the separation of N2O from agas stream containing at least N₂O and nitrogen, said process comprisingpassing said gas stream over a bed of adsorbent having a workingcapacity ΔN₂O of greater than or equal to 3.56×10⁻⁴ at IBL.

In a preferred embodiment the gas stream is air.

In a preferred embodiment the adsorbent is selected from the groupconsisting of clinoptilolite, chabazite and Li-exchanged zeolite orcombinations thereof.

In a preferred embodiment the gas stream contains less than 100 ppm N₂O.

The invention also comprises an adsorption apparatus for the removal ofN₂O from a gas containing N₂O, nitrogen and other components, saidapparatus comprising one or more beds of at least a first adsorbent,wherein said first adsorbent is an N₂O selective adsorbent selected fromthe group consisting of clinoptilolite, chabazite and Li-exchangedzeolite.

In a preferred embodiment the other components in the gas include H₂Oand CO₂, and said apparatus further contains one or more additionaladsorbents for the adsorption of H₂O and CO₂, and wherein the additionaladsorbents are upstream of said first adsorbent.

The process and apparatus of the present invention provide surprisinglysuperior N₂O removal efficiency over prior art processes and systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of preferred embodiments and theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of the breakthrough test apparatus;

FIG. 2 is a graph of N₂O breakthrough curves for NaX(2.5) for N₂O/N₂ andN₂O/He;

FIG. 3 are nitrogen isotherms for certain adsorbents;

FIG. 4 are breakthrough curves to show initial breakthrough (0.05 ppmlevel);

FIG. 5 illustrates IBL N₂O loading for certain adsorbents (IBL:Initialbreakthrough loading—N₂O adsorbed per unit weight of adsorbent at the 50ppb breakthrough);

FIG. 6 is a schematic of an adsorption system useful for practicing theinvention.

FIG. 7 a illustrates a bed from a conventional prepurifier with an addedlayer for N₂O removal in accordance with the invention.

FIG. 7 b illustrates a bed from a prepurifier with a first layer forwater removal and a downstream mixed layer for CO₂ and N₂O removal.

FIG. 7 c illustrates a bed from a prepurifier with a first layer forwater removal and a downstream mixed layer for hydrocarbon and N₂Oremoval.

FIG. 7 d illustrates a bed from a prepurifier with a first layer forwater removal a second layer for CO₂ removal, a third layer for removalof hydrocarbons and a final layer for N₂O removal.

DETAILED DESCRIPTION OF THE INVENTION

The invention is based upon first recognizing the critical components(N₂O/N₂) to be separated, then isolating these critical components in anadsorption zone within the adsorber and finally selecting an adsorbentthat can efficiently affect the separation.

The general problem to be solved is the removal of ppb levels (≈350-400ppb) of N₂O from a mixture of air containing other contaminants(including at least CO₂ and H₂O) prior to air separation by cryogenicmeans.

In a process for removal of contaminants from a gas mixture byadsorption, it is common to adsorb contaminants successively in theorder of decreasing adsorptivity and/or decreasing selectivity withrespect to a chosen adsorbent. The effectiveness of such a process canoften be improved by using a combination of adsorbents, configured inlayers or mixtures, to enhance the removal of each contaminant, i.e. byselecting particular adsorbents to achieve maximum adsorptivity and orselectivity of each contaminant relative to the gas mixture. The use ofdifferent adsorbents disposed in layers in the adsorber is well known inthe art.

The selection of an adsorbent to remove a particular contaminant dependsupon many factors, e.g. the type and composition of both the targetedcontaminant and other gases in the mixture at the point of removalwithin the adsorber, the relative selectivity of the adsorbent for thecontaminant(s) and non-contaminants, and the loading capacity of theadsorbent for the contaminant.

In a preferred embodiment of the invention, an adsorbent bed is firstconfigured to remove substantially all of the CO₂ and H₂O from the feedstream (e.g. air) prior to removing N₂O. N₂O is then subsequentlyremoved the partially purified feed stream. The present inventiondiffers from the prior art in that an adsorbent is selected for the N₂Oseparation such that the adsorbent has both a high ΔN₂O/ΔN₂ separationfactor and a high ΔN₂O (in the presence of high N₂ concentrations)capacity. Natural clinoptilolite, natural chabazite and LiX arepreferred embodiments for N₂O removal from gases, particularly air, inaccordance with the invention. With the addition of the N₂O adsorbent,the combination of all the adsorbents in the bed removes at least 90%and preferably all of the N₂O from the feed stream. Thus, the purifiedstream contains preferably less than 100 ppb (parts per billion), morepreferably less than 50 ppb and most preferably less than 10 ppb N₂O atthe local stream conditions

Thus, this invention provides a simple and efficient way tosubstantially remove all of N₂O prior to cold box in cryogenic airseparation plants, thereby ensuring a safe operation and potentiallyreducing liquid oxygen drainage.

The results of the present invention are even more surprising whenevaluated in the context of the prior art selection criteria, i.e. theratio of Henry Law constants for N₂O/CO₂>0.49 (U.S. Pat. No. 6,106,593).The ratio of Henry Law constants (ratio of initial isotherm slopes forN₂O/CO₂) was computed for clinoptilolite.

This ratio was found to be about 0.40, well below the minimumrecommended value of 6,106,593.

The prior art solution to the problem of removing N₂O from air hasfocused upon finding an adsorbent with high N₂O/CO₂ selectivity;however, this is difficult DUE to the similar electrostatic propertiesof these two adsorbates. Further electrostatic considerations do notrecognize the significant effects of the relative concentrations of theadsorbates; e.g. in ambient air, CO₂ concentrations are typically 350ppm to 400 ppm, one thousand times that of N₂O concentrations.

In the present invention because H₂O and CO₂ are almost completelyremoved prior to N₂O removal, the relevant separation is the removal ofN₂O from N₂. In this case, while electrostatics favor the selectivity ofN₂O, the significant concentration difference (˜790,000 ppm N₂ comparedto ˜350 ppb N₂O) favors the adsorption of N₂. Thus in accordance withthe present invention the adsorbent required has high N₂O/N₂selectivity, low N₂ working capacity and sufficient N₂O working capacityto satisfy the purification requirements.

In the practice of the present invention, adsorbent performance may beestimated by determining the working capacity of each of the primaryadsorbates, i.e. N₂ and N₂O. The separation factor α, as defined belowis utilized to evaluate the adsorbent effectiveness. This methodology isdiscussed in detail in U.S. Pat. No. 6,152,991. $\begin{matrix}{\alpha = {\frac{\Delta\quad N_{2}O}{\Delta\quad N_{2}} = \frac{{w_{N_{2}O}( {y,p,T} )}_{ads} - {w_{N_{2}O}( {y,p,T} )}_{des}}{{w_{N_{2}}( {y,p,T} )}_{ads} - {w_{N_{2}}( {y,p,T} )}_{des}}}} & (1)\end{matrix}$where separation factor a is defined as the ratio of the workingcapacities. The numerator in this equation is the working capacity ofN₂O, which is equal to the difference in loading w between adsorptionand desorption conditions. The adsorption and desorption conditions arecharacterized by composition y, pressure p and temperature T.

In TSA air prepurification, maximum regeneration temperatures may varyfrom about 100° C. to about 350° C. As a result, it is expected that theadsorbates (particularly atmospheric gases) will be completely thermallydesorbed. Under such conditions, Equation (1) can be simplified asfollows: $\begin{matrix}{\alpha = {\frac{\Delta\quad N_{2}O}{\Delta\quad N_{2}} = \frac{{w_{N_{2}O}( {y,p,T} )}_{ads}}{{w_{N_{2}}( {y,p,T} )}_{ads}}}} & (2)\end{matrix}$

When a contaminant is removed in a shallow adsorbent layer in TSA andsignificant resistance to mass transfer exists, the selectivity isredefined according to Equation (3): $\begin{matrix}{\frac{\Delta\quad X_{A}}{\Delta\quad X_{B}} = \frac{\frac{m_{in}}{w_{s}}{\int_{0}^{t_{b}}{( {y_{in} - y_{out}} )\quad{\mathbb{d}t}}}}{{X_{B}( {y,P,T} )}_{ADS}}} & (3)\end{matrix}$The numerator in Equation (3) represents the working capacity of theadsorbent for the contaminant. m_(in) represents the molar feed flowinto the bed, y_(in) and y_(out) are the inlet and outlet mole fractionsof the minor component, respectively, w_(s) is the mass of adsorbent andtb is the breakthrough time corresponding to a predeterminedconcentration. The denominator is the equilibrium capacity of the majorcomponent at the conditions at the end of the adsorption step, i.e.assuming complete desorption of all components. This situation mayresult when using small pore zeolites at conditions where the depth ofthe adsorbent layer is shorter than the mass transfer zone length.

This method is superior to prior art methods for evaluating theselectivity of N₂O in that the working capacities are determined at thepartial pressure of each individual component at the relevant processconditions. Furthermore, coadsorption effects are incorporated in thedetermination of the loadings. The analysis can be performed usingeither a multicomponent isotherm model supported by pure component data(e.g. loading ratio correlation isotherm model) or directly fromexperimental data. Since the concentration of N₂ is overwhelmingcompared to N₂O, the coadsorption effect of N₂O upon N₂ is negligible.Thus, the denominator of Equation (2) or Equation (3) may be obtaineddirectly from the measured pure-component N₂ isotherm.

Conversely, the coadsorption of N₂ has a very significant effect uponthe adsorption of N₂O. If accurate low concentration pure-componentisotherm data for N₂O is available or attainable, then Equation (2) maybe applied to assess working capacity and selectivity. Otherwise, it ispreferred to determine the working capacity for N₂O directly undercoadsorption conditions with N₂ using the breakthrough test method,which is well know to those skilled in the art. This allows any kineticeffects to be incorporated into the working capacity as well. Thebreakthrough tests allow for the determination of the equilibriumcapacity of a component at saturation, and the breakthrough capacity andtime at some defined breakthrough level, e.g. 50 ppb.

In order to evaluate adsorbents for N₂O working capacity and separationfactor according to Equation (3), a breakthrough test apparatus wasconstructed as shown in FIG. 1.

The adsorbents tested were obtained from the sources listed below. Thenatural adsorbents (clinoptilolite and chabazite) were obtained fromSteelhead Specialty Minerals, WA. Synthetic zeolites were obtained fromvarious manufacturers: Zeolyst (ZSM5, mordenite), Zeochem (CaX(2.5)and >85% Ca) and UOP (13X, NaX(2.3), LiX(2.3) >97% Li; LiX(2.0) >97% Li,NaY). Note that the numbers recited in the parentheses (e.g. 2.5, 2.3,and 2.0) refer to SiO₂/Al₂O₃ ratio. All of the adsorbents were thermallyregenerated at 350° C., 1.0 bar pressure and under N₂ purge forapproximately 16 hours before each test. After regeneration theadsorbents were allowed to cool to the test temperature of 27° C.

Breakthrough tests were conducted using the following feed gas mixtures:1.0 ppm N₂O in N₂ and 1.0 ppm N₂O in He. Also, N₂ isotherms weredetermined gravimetrically. The results from these tests were examinedto determine N₂O separation factor and working capacity. Tests wereperformed to saturation, i.e. until the effluent N₂O concentrationreached the feed level concentration. For evaluating various adsorbents,a N₂O concentration of 1.0 ppm was selected. All of the breakthroughtests were performed at 6 bar, 300 K and an inlet gas flowrate ofapproximately 21.3 slpm (0.08 kmol/m² s) using an adsorption columnlength of either 22.9 cm or 5.6 cm. The feed conditions arerepresentative of conditions at the inlet of an air prepurifier for atypical cryogenic air separation plant. Breakthrough curves were alsogenerated using 1.0 ppm N₂O in He and 1.0 ppm CO₂ in He to determinepure-component N₂O or CO₂ loadings, respectively. In order to determinethe coadsorption effects of N₂O and CO₂ in N₂, breakthrough tests werealso performed using 1.0 ppm N₂O+1.0 ppm CO₂ in N₂. Initial breakthroughwas established at 50.0 ppb N₂O and initial breakthrough loading (IBL)was determined as the average amount of N₂O adsorbed per unit weight ofadsorbent at the 50.0 ppb breakthrough.

Breakthrough tests are conducted in the following manner using theapparatus shown in FIG. 1. A challenge gas from source 1 containing thecontaminant(s) of interest (e.g. 10 ppm N₂O in N₂) is metered throughflow controller 3, and mixes in a gas mixer 5 with high purity diluentN₂ or He from source 2 and provided at a prescribed flowrate through aflow controller 4 to achieve the desired feed concentration ofcontaminant(s). This mixed challenge gas is then fed to the test bed 6containing the adsorbent. The effluent is passed through a flow meter 7to the N₂O analyzer 8 (TEI Model 46-C) where the breakthroughconcentration of N₂O is monitored as a function of time. Control valve 9is used to maintain the desired pressure in the system. The piping andadsorbent bed are maintained at the same temperature as the feed byimmersing them in a thermostat bath (not shown).

The following are non-limiting examples that illustrate the methodologyfor selecting adsorbents and their implementation in accordance with theinvention.

EXAMPLE 1 N₂ Coadsorption Effects

Adsorbents were tested as described above to determine the effect of N₂coadsorption upon the adsorption of N₂O. The results of the saturationcapacity of N₂O (1.0 ppm) in N₂ and in He on various adsorbents arecompared in Table 1 below. These results were determined for feedconditions of 6.0 bar, 300° K. and 0.08 kmol/m² s molar flux using a22.9 cm or 5.6 cm adsorbent bed length. The SiO₂/Al₂O₃ ratio isspecified for some adsorbents in the table, e.g. NaX (SiO₂/Al₂O₃=2.3).FIG. 2 shows N₂O breakthrough curves for adsorbent 13X (NaX 2.5) for N₂Oin N₂ and N₂O in He. TABLE 1 Effect of N₂ Coadsorption Upon N₂O LoadingN₂O loading N₂O loading (mmol/gm) (mmol/gm) 1.0 ppm 1.0 ppm PreferredMaterial N₂O/He N₂O/N₂ Adsorbents NaY 4.06 × 10⁻⁴ 1.42 × 10⁻⁴ NaZSM5 1.7 × 10⁻³ 3.59 × 10⁻⁴ NaKX 4.58 × 10⁻⁴ 13X NaX (2.5) 9.55 × 10⁻⁴ 6.09× 10⁻⁴ NaX (2.3) 1.73 × 10⁻³ 7.03 × 10⁻⁴ CaX 1.98 × 10⁻³ Na-Mordenite6.38 × 10⁻³ Clinoptilolite (CS400) 8.86 × 10⁻² 3.60 × 10⁻³ X LiX (2.3) 5.5 × 10⁻³ 1.22 × 10⁻³ X LiX (2.0)  6.7 × 10⁻³ 1.74 × 10⁻³ X Chabazite6.75 × 10⁻² 3.42 × 10⁻³ X Clinoptilolite (TSM140) 6.84 × 10⁻² 8.20 ×10⁻³ X

These results clearly show the substantial effect of N₂ coadsorption,resulting in a decrease in N₂O capacity from 36% to 96% compared to thesingle component saturation capacity (1.0 ppm N₂O in He). The fourthcolumn of Table 1 also indicates examples of preferred adsorbents of theinvention (i.e. adsorbents with high N₂O loading in the presence of N₂).

EXAMPLE 2 N₂ Isotherms

Isotherms for N₂ at 300° K. were determined for various adsorbents overa range of pressures which included typical feed pressures to aprepurifier of a cryogenic air separation unit. Example isotherms areshown in FIG. 3. The pure component N₂ loadings from these isotherms arecompared in Table 3 for various adsorbents at 6.0 bar. TABLE 2Equilibrium Loadings of N₂ at 6.0 bar, 300° K Material N₂ loading(mmol/gm) Silicalite 0.68 H-ZSM5 0.74 4A 1.11 NaY 0.85 NaZSM5 NaKX 0.8613X NaX (2.5) 1.30 NaX (2.3) 1.33 CaX 1.53 Na-Mordenite 1.40Clinoptilolite (CS400) 0.64 LiX (2.3) 1.71 LiX (2.0) 2.29 Chabazite 1.22Clinoptilolite (TSM140) 1.23

EXAMPLE 3 Breakthrough at 50 ppb N₂O

The average N₂O loading (IBL) of the adsorbent bed was determined at50.0 ppb N₂O from the same N₂O/N₂ tests reported in Table 1 above.Typical breakthrough results are illustrated in FIG. 4. The IBL values(reflecting not only the 50 ppb breakthrough, but also N₂ coadsorption)are compared for various adsorbents in Table 3 and shown in FIG. 5.Separation factors were also computed using Equation 3. The N₂ loadingsat 6 bar from Table 2 are used in Equation 3 to represent the workingcapacity of N₂ in the process.

N₂O working capacities can be computed either from the saturationloadings of N₂O/N₂ or from the average loadings of N₂O at the IBL. Theseparation factor computed from the average loadings at the IBL is thepreferred method since it reflects both the equilibrium and dynamiceffects of the adsorbent. Nevertheless, the former method (whichreflects equilibrium effects only) is acceptable when only isotherms andno breakthrough data are available. Both separation factors aretabulated in Table 3 for various adsorbents. Both methods identifyclinoptilolite (TSM-140) as having the highest N₂O/N₂ separation factor,as well as establishing the same group of six adsorbents with higherseparation factors compared to the prior art choice of CaX. The order ofeffectiveness amongst the adsorbents in the preferred group is affectedby which separation factor method is used.

Using the methodology of the invention, clinoptilolite TSM-140 is themost preferred adsorbent for removing N₂O from air. This adsorbent hasthe highest N₂O working capacity at IBL, the highest working N₂O/N₂separation factor and a moderate N₂ working capacity. It is evident fromTable 3 that TSM140 has approximately 6.6 times the average N₂Obreakthrough capacity of CaX. Thus, clinoptilolite TSM140 provides asurprisingly superior solution to the problem. TABLE 3 IBL and α forVarious Adsorbents α α IBL (Eqn 3) (Eqn 3) Material (mmol/gm) N₂O @ IBLN₂O @ 1.0 ppm Silicalite 2.20 × 10⁻⁵ 3.24 × 10⁻⁵ 1.11 × 10⁻⁴ Activatedcarbon 2.60 × 10⁻⁵ H-ZSM5 3.60 × 10⁻⁵ 4.89 × 10⁻⁵ 1.85 × 10⁻⁴ 4A 4.30 ×10⁻⁵ 3.87 × 10⁻⁵ NaY 5.20 × 10⁻⁵ 6.09 × 10⁻⁵ 1.67 × 10⁻⁴ NaZSM5 6.50 ×10⁻⁵ NaKX 1.32 × 10⁻⁴ 1.54 × 10⁻⁴ 5.33 × 10⁻⁴ 13X NaX (2.5) 1.87 × 10⁻⁴1.44 × 10⁻⁴ 4.68 × 10⁻⁴ NaX (2.3) 2.27 × 10⁻⁴ 1.71 × 10⁻⁴ 5.29 × 10⁻⁴CaX 2.40 × 10⁻⁴ 1.57 × 10⁻⁴ 1.29 × 10⁻³ Na-Mordenite 3.13 × 10⁻⁴ 2.24 ×10⁻⁴ 4.56 × 10⁻³ Clinoptilolite 3.56 × 10⁻⁴ 5.57 × 10⁻⁴ 5.64 × 10⁻³(CS400) LiX (2.3) 3.82 × 10⁻⁴ 2.23 × 10⁻⁴ 7.10 × 10⁻⁴ LiX (2.0) 7.68 ×10⁻⁴ 3.35 × 10⁻⁴ 7.61 × 10⁻⁴ Chabazite 1.04 × 10⁻³ 8.55 × 10⁻⁴ 2.81 ×10⁻³ Clinoptilolite 1.58 × 10⁻³ 1.28 × 10⁻³ 6.64 × 10⁻³ (TSM140)

EXAMPLE 4 N₂O and CO₂ Coadsorption Effects

In the present invention, the adsorber is preferably configured so thatthe water vapor and CO₂ contaminants in the feed air are substantiallyremoved from the gas mixture prior to the final clean up of the streamin which N₂O is adsorbed. Sufficient removal of N₂O must be affected toprevent breakthrough beyond 50 ppb N₂O during the adsorption step of theprepurifier cycle. In this situation, it is estimated that low levels ofCO₂ (less than 10.0 ppm, most likely less than 1.0 ppm) could be presentwith 100 ppb or more N₂O with the remaining bulk gas being of aircomposition (N₂/O₂). In order to verify the effectiveness of theadsorbent and to determine the competitive coadsorption effects of CO₂upon N₂O, breakthrough tests were performed with clinoptilolite TSM140using a feed mixture with 1.0 ppm CO₂ and 1.0 ppm N₂O in N₂. The resultsare shown in Table 4 for the average IBL loading and saturated loadingsof N₂O. The loadings are comparable within the experimental error withthe N₂O loadings from Tables 1 and 3. Thus, it is evident that CO₂ andN₂O do not compete with each other at these low concentrations, i.e.each competes individually only with N₂. This can be explained by thefact that the number of adsorbed molecules of N₂ (Table 3) is fargreater than either those adsorbed molecules of N₂O or CO₂. As N₂O orCO₂ enter the adsorbent the adsorption sites are predominantly occupiedby N₂ molecules. TABLE 4 IBL and saturation loadings (1 ppm N₂O + 1 ppmCO₂ in N₂) IBL N₂O N₂O saturation loading (mmol/gm) (mmol/gm) 1.6 × 10⁻³8.3 × 10⁻³

These results suggest that other low-level contaminants present in thegas stream can also be removed simultaneously with N₂O. In the case ofclinoptilolite, adsorbates with a kinetic diameter less than about 4.5 Åand with an interaction potential energy greater than that of N₂ arelikely to completely or partially removed. Such adsorbates include butare not limited to acetylene, ethylene and propane.

EXAMPLE 5 Prepurifier Operation

A two-bed TSA prepurifier was designed to evaluate the adsorbentrequirement for complete N₂O removal. The inlet air flow was 569,000NCFH at a pressure of 72 psig, and an ambient temperature of about 78 F.The ambient N₂O level was about 400 ppb and the CO₂ level was about 400ppm. Average temperature of air going into the prepurifier was about44.6 F. Each bed has an internal diameter of 8.0 ft.

Initially the bed has two layers: First layer of Alumina (9.9 in) andthe second layer of 13X APG (46.5 in). The breakthrough level of N₂O wasmonitored for this two-layered bed system. It was found that about 80%of the entering N₂O in each cycle is retained in the bed. About 20% ofthe incoming N₂O breaks through the bed. The thickness of theclinoptilolite TSM 140 layer needed to ensure complete N₂O removal ifthis layer is added downstream of 13 X layer was calculated to be about9 in. Therefore, a very thin layer of clinoptilolite TSM140 addeddownstream of 13X layer substantially eliminates the problem of N₂Oleaking into the cold box. The resulting prepurifier with the threelayers could substantially eliminate all of water vapor, CO₂ and N₂O andmost of the hydrocarbons entering the bed.

The Si/Al ratio and the composition (% of exchangeable cations) of majorcations in clinoptilolite TSM140 and CS400 are given in Table 5. It isevident from the working capacity in Table 1 and the selectivity inTable 3 that TSM140 has superior ability to remove N₂O in the presenceof high concentrations of N₂ compared to CS400. Although both materialsare of clinoptilolite structure with nearly the same Si/Al ratio, theefficiency of adsorption of N₂O is quite different. A primary differencein the composition of these materials is in the amount of Na cationpresent. We have found that a preferred clinptilolite would have sodiumin an amount between about 30 to about 80% of the exchangeable cations.TABLE 5 Composition of natural adsorbents TSM140 CS400 Si/Al 4.84 4.78 %Ca 12 34 % Na 62 14 % K 19 32 % Mg 7 20

As indicated above, clinoptilolite (most preferably TSM140 and CS400),chabazite and LiX (most preferably having a greater than 86% Li exchangeand a SiO₂/Al₂O₃ ratio of (2.3) or (2.0)) were found to be the preferredadsorbents for removing N₂O from air prior to cryogenic air separation.

The natural zeolites clinoptilolite and chabazite, having higher Si/Alratio (3.0 to 5.0 in this invention) than type X zeolites, are “weaker”adsorbents compared to LiX, CaX and NaX. These natural zeolites alsohave a smaller micropore volume than type X. These factors contribute tothe lower N₂ adsorption capacity and selectivity; however, overallcapacity is typically not a critical issue when removing tracequantities of contaminants. Conversely, because of the higher Si/Alratio of clinoptilolite and chabazite, these materials have fewercations. This generally means weaker energetics in relation to polaradsorbates. While this favors weaker attraction for N₂ it also meansweaker attraction for N₂O as well.

The adsorption characteristics of zeolites are strongly dependent upontheir cation composition. Both the equilibrium and kinetic adsorptionproperties can be altered by ion exchange. Cation type, location andnumber can completely alter adsorption behavior. Acid washing of smallpore natural zeolites may remove impurities that block the pores,progressively eliminate cations and finally dealuminate the structure asthe strength and duration of the treatment increases. Alkali washing hasbeen shown to modify both the pore size and pore volume ofclinoptilolite. The method and extent of dehydration is important indetermining the adsorption properties and structural stability ofactivated zeolites. Dehydration and thermal treatment can result incation migration, thereby influencing cation location and pore openings.Any of the methods may be used to further improve the adsorptioncharacteristics of the preferred adsorbents of this invention, i.e.equilibrium and/or kinetic adsorption properties may be effected.

As indicated previously, the invention relates to the removal of N₂Ofrom air in which the H₂O and CO₂ have already been substantiallyremoved. By “substantially removed” we mean removed to levels of lessthan 10 ppm, preferably less than 1.0 ppm. However, the invention may beapplied in removing N₂O in the presence of higher concentrations of CO₂and/or H₂O. In the most direct application of the invention, a layer ofthe N₂O-selective adsorbent is located downstream (as determined by thedirection of feed flow during adsorption) of those adsorbents that areused to remove H₂O and CO₂ Typical TSA prepurifiers have either a singlelayer of zeolite to remove both H₂O and CO₂ or a layer of activatedalumina (for H₂O removal) followed by a layer of zeolite for CO₂adsorption. In general, adsorbents useful for the water and CO₂adsorption are known to those skilled in the art and include cationcontaining zeolites (synthetic or natural), activated alumina, silicagel and activated carbon.

In such configurations, the N₂O-selective layer would represent either asecond or third layer of adsorbent, respectively, in the prepurifieradsorber. Other alternative configurations contemplated by thisinvention are described below. According to the invention N₂O isadsorbed onto at least one adsorbent selected from naturalclinoptilolite, natural chabazite and Li exchanged X zeolite. Theremoval of N₂O from the gas stream is achieved by passing the gas streamthrough a bed of clinoptilolite, chabazite or LiX or a mixture of thesein the temperature range of about −70° C. to 80° C., preferably 0° C. to40° C. While the invention is directed at the removal of lowconcentrations of N₂O from air, it may also be used to remove higherconcentrations of N₂O from air or other gas mixtures. Typical gases thatcan be purified by this process include air, nitrogen, oxygen, argon,methane etc.

The process of N₂O removal is carried out preferably in a cyclic processsuch as pressure swing adsorption (PSA), temperature swing adsorption(TSA), vacuum swing adsorption (VSA) or a combination of these. Suchprocesses can be used for removing ppm or ppb levels of N₂O present inair prior to cryogenic separation. The process of the invention may becarried out in single or multiple adsorption vessels operating in acyclic process that includes at least the steps of adsorption andregeneration. The adsorption step is carried out at pressure range of1.0 to 25 bar and preferentially from about 3 to 15 bar. The temperaturerange during the adsorption step is −70° C. to 80° C. When a PSA processis used, the pressure during the regeneration step is lower than theadsorption pressure, preferably in the range of about 0.20 to 10.0 bar,and preferably 1.0 to 2.0 bar. For a TSA process, regeneration iscarried out at a temperature greater than the adsorption temperature;preferably in the range of about 50° C. to 400° C., more preferablybetween 100° C. to 300° C. In cryogenic air separation processes, theregeneration gas is typically taken from the product or waste N₂ or O₂streams.

In the cyclic process, the gas containing N₂O is introduced at one endof an adsorption vessel that contains at least a layer of N₂O-selectiveadsorbent. As the gas passes through the bed, N₂O is adsorbed and anessentially N₂O-free gas is obtained at the other end of the bed. As theadsorption step proceeds, a N₂O front develops in the bed and movesforward through the bed during the adsorption step. When the frontreaches the end of the bed, which is determined by the concentration ofN₂O acceptable in the outlet gas, the adsorption step is terminated andthe vessel enters the regeneration mode. The method of regenerationdepends upon the type of cyclic process. For a PSA process, generallythe vessel is countercurrently depressurized. Subatmospheric pressurelevels can be additionally employed during the regeneration steps usinga vacuum pump. For a TSA process, regeneration of the adsorbent bed isachieved by passing heated gas countercurrently through the bed. Usingthe thermal pulse method, a cooling purge step follows the hot purgestep. The heated regeneration gas may also be provided at a reducedpressure (relative to the feed) so that a combined TSA/PSA process isaffected. For removal of N₂O from air, the TSA/PSA method is preferred.

In some cases, passing an inert or weakly adsorbed purge gascountercurrently through the bed can further clean the adsorbent bed. Ina PSA process, the purge step usually follows the countercurrentdepressurization step. In a TSA process, the heated purge gas can beused for regeneration of adsorbent. In case of a single vessel system,the purge gas can be introduced from a storage vessel, while formultiple bed system, purge gas can be obtained from another adsorberthat is in the adsorption phase.

The adsorption system can have more steps than the two basic fundamentalsteps of adsorption and desorption. For example, top to top equalizationor bottom to bottom equalization can be used to conserve energy andincrease recovery.

In a specific embodiment, the prepurification process operates asfollows with reference to FIG. 6. Referring to FIG. 6, feed air fed tothe system via conduit 23 is compressed in compressor 10 and cooled bychilling means 11 prior to entering one of two adsorbers (16 and 17)where at least the contaminants H₂O, CO₂ and N₂O are removed from theair. The purified air exits the adsorber via conduit 24 and then entersthe air separation unit (ASU) (not shown) where it is then cryogenicallyseparated into its major components N₂ and O₂. In special designs of theASU, Ar, Kr and Xe may also be separated and recovered from the air.While one of the beds is adsorbing the contaminants from air, the otheris being regenerated using purge gas provided via conduit 25. A dry,contaminant-free purge gas may be supplied from the product or wastestream from the ASU or from an independent source to desorb the adsorbedcontaminants and thereby regenerate the adsorber and prepare it for thenext adsorption step in the cycle. The purge gas may be N₂, O₂, amixture of N₂ and O₂, air or any dry inert gas. In the case of thermalswing adsorption (TSA), the purge gas is first heated in heater 22 priorto being passed through the adsorber in a direction countercurrent tothat of the feed flow in the adsorption step. TSA cycles may alsoinclude a pressure swing. When only pressure swing adsorption (PSA) isutilized, there is no heater.

The operation of a typical PSA cycle is now described in reference toFIG. 6 for one adsorber. One skilled in the art will appreciate that theother adsorber vessel will operate with the same cycle, only out ofphase with the first adsorber in such a manner that purified air iscontinuously available to the ASU. Feed air is introduced via conduit 23to compressor 10 where it is pressurized. The heat of compression isremoved in chilling means 11, e.g. a mechanical chiller or a combinationof direct contact after-cooler and evaporative cooler. The pressurized,cool and H₂O-saturated feed stream then enters adsorber 16. Valve 12 isopen and valves 14, 18 and 20 are closed as the adsorber vessel 16 ispressurized. Once the adsorption pressure is reached, valve 18 opens andpurified product is directed to the ASU with conduit 25 for cyrogenicair separation. When the adsorber 16 has completed the adsorption step,valves 18 and 12 are closed and valve 14 is opened to blow down theadsorber 16 to a lower pressure, typically near ambient pressure. Oncedepressurized, valve 20 is opened and heated purge gas is introducedinto the product end of the adsorber 16. At some time during the purgecycle, the heater is turned off so that the purge gas cools the adsorberto near the feed temperature.

One of ordinary skill in the art will further appreciate that the abovedescription represents only an example of a typical prepurifier cycle,and there are many variations of such a typical cycle that may be usedwith the present invention. For example, PSA may be used alone whereinboth the heater 22 and the chilling means 11 may be removed.Pressurization may be accomplished with product gas, feed gas or acombination of the two. As indicated above, bed-to-bed equalization mayalso be used and a blend step may be incorporated where a freshlyregenerated bed is brought on line in the adsorption step with anotheradsorber nearing completion of its adsorption step. Such a blend stepserves to smooth out pressure disturbances due to bed switching and alsoto minimize any thermal disturbances caused when the regenerated bed isnot completely cooled to the feed temperature. Furthermore, theinvention may be practiced with a prepurifier cycle not limited to twoadsorber beds.

As indicated above, the most preferred embodiment of the presentinvention is the removal of trace amounts of N₂O from gaseous streams,particularly from air prior to cryogenic separation. The method of theinvention is particularly applicable to the removal of low tointermediate (e.g. ppb to ppm) concentrations of N₂O from a feed stream.For example, the methodology is particularly useful in airprepurification (prior to cryogenic distallation) where the N₂Oconcentration is on the order of 350 ppb. The adsorbents are alsoespecially effective for the removal of N₂O from gas streams containing100 ppm or less N₂O. If the gas contains water vapor, this should mostpreferably be removed to a level of less than 100 ppb prior to passingit through the N₂O adsorbent. If the gas contains CO₂, CO₂ should beremoved to levels less than 10 ppm, preferably less than 1 ppm, however,removal of CO₂ is not as essential as removal of water vapor. Therelative thickness of the N₂O layer depends upon the pressure,temperature, composition and flow of the feed gas and the desired purityof the purified gas, but could be determined by one of ordinary skill inthe art.

As indicated above, in a preferred air prepurification embodiment of theinvention, water vapor and CO₂ are substantially removed from air on atleast one layer of activated alumina or zeolite, or by multiple layersof activated alumina and zeolite prior to passing the air stream throughthe N₂O adsorbent layer. Optionally, the N₂O-selective adsorbent layermay be extended and used to remove part or all of the CO₂ from air.Alternately, in an adsorption vessel, a first layer of alumina can beused to remove water vapor and a next layer comprised of a mixture ofthe N₂O-selective adsorbent and 13X (or other zeolite) can be used toremove both CO₂ and N₂O from the air. Such an adsorbent mixture may becomposed of physically separate adsorbents or of different adsorbentsbound together in the form of a composite. Additionally, the N₂Oslective adsorbent may be deposited in the form of fine particles on asubstrate such as a monolith.

In the existing prepurifier beds with water adsorbent layer and CO₂adsorbent layer, the method of the invention allows for replacing10-100% of CO₂ adsorbent layer with the N₂O adsorbent at the mostdownstream end.

As illustrated in FIGS. 7 a and 7 b various layered arrangements of abed 30 are possible. FIG. 7 a shows for example a bed 30 arrangementcomprising a layer of a. first adsorbent for water removal 31; a layer32 of a second adsorbent for CO₂ removal; and a third layer 33 that isthe N₂O adsorbent.

In FIG. 7 b, a bed 30 arrangement having a first layer of wateradsorbent 31′ and a second layer 34 that is a mixture or composite of aCO₂ adsorbent and the N₂O adsorbent

Some chemically modified forms of the adsorbents used in the process ofthis invention would also be appropriate for N₂O removal purposes. Thus,the method of the invention can be carried out wherein saidclinoptilolite and chabazite are natural or synthetic, and haveexchangeable cations from ions of Group 1A, Group 2A, Group 3A, Group3B, the lanthanide series of the Periodic Table, as well as mixtures ofthese. According to the invention, at least nitrous oxide gas containedin a gas stream is separated, whereby N₂O is adsorbed on at least oneadsorbent or a mixture of these selected from the following: naturalclinoptilolite, natural chabazite and Li exchanged X zeolite. Themixture can be made with any ratio of these adsorbents namely 0-100%clinoptilolite, 0-100% chabazite and 0-100% LiX, wherein the total ofthese is 100%.

The adsorbent beds used in the method of the invention can have varietyof configurations such as vertical beds, horizontal beds or radial bedsand can be operated in a pressure swing adsorption mode, temperatureswing adsorption mode, vacuum swing adsorption mode or a combination ofthese.

Clinoptilolite has excellent thermal stability at very high temperaturesup to 700° C. Thus, it can be regenerated at very high temperatures ifneeded.

Since clinoptilolite and chabazite are natural zeolites mined from theearth, they should be thermally treated before being used in the methodof the invention. These natural zeolites should also be ground to asuitable average grain size, for example, 4 to 50 mesh, preferably 8 to12 mesh (US Sieve Series), although smaller or larger average sizes maybe employed depending upon the requirements of the application.

In the method of the invention, the ground natural mineral withpredetermined grain size distribution is thermally treated at atemperature of 250° C. to 700° C. It is important to dehydrate thezeolites to less than 1.0 % (wt) H₂O. Those skilled in the art arefamiliar with such sizing and calcination procedures.

The adsorbents in this method may be shaped by a series of methods intovarious geometrical forms such as beads and extrudates. This mightinvolve addition of a binder to zeolite powder in ways very well knownto prior art. These binders might also be necessary for tailoring thestrength of the adsorbents. Binder types and shaping procedures are wellknown to prior art and the current invention does not put anyconstraints on the type and percentage amount of binders in theadsorbents.

The N₂O adsorbent could also potentially adsorb some hydrocarbons fromair. To ensure complete removal of hydrocarbons, the N₂O adsorbent canbe physically mixed in a layer 35 with a hydrocarbon selective adsorbentsuch as 5A (Bed 30 in FIG. 7 c). Alternately, an additional layer ofhydrocarbon selective adsorbent can be placed upstream (layer 36) ordownstream (not shown) of the N₂O adsorbent layer 33′ (Bed 30 in FIG. 7d). Note that 31″ and 31′″ refer to a layer of water adsorbent and 32′and 32″ refer to a layer of CO₂ adsorbent in FIGS. 7 c and 7 drespectively.

The term “comprising” is used herein as meaning “including but notlimited to”, that is, as specifying the presence of stated features,integers, steps or components as referred to in the claims, but notprecluding the presence or addition of one or more other features,integers, steps, components, or groups thereof.

Specific features of the invention are shown in one or more of thedrawings for convenience only, as each feature may be combined withother features in accordance with the invention. Alternative embodimentswill be recognized by those skilled in the art and are intended to beincluded within the scope of the claims.

1. An adsorption process for the removal of N₂O from a gas containingN₂O, nitrogen and other components, said process comprising passing saidgas over a bed of one or more adsorbents and producing a purified gas,wherein said one or more adsorbents is selected from the groupconsisting of clinoptilolite, chabazite and Li-exchanged zeolite.
 2. Theprocess of claim 1, wherein said gas is air.
 3. The process of claim 1,wherein said other components include water and CO₂.
 4. The process ofclaim 1, wherein said N₂O is in said gas in an amount of less than 100ppm.
 5. The process of claim 3, wherein said water and said CO₂ areadsorbed on an additional adsorbent prior to said gas passing over saidclinoptilolite, chabazite or said Li-exchanged zeolite.
 6. The processof claim 1, wherein said gas is air.
 7. The process of claim 1, whereinsaid process is either pressure swing adsorption or temperature swingadsorption.
 8. The process of claim 1, wherein said process is acombination of temperature swing adsorption and pressure swingadsorption.
 9. The process of claim 5, wherein at least 90% of the N₂Oin said gas is adsorbed.
 10. The process of claim 1, wherein saidadsorbent is clinoptilolite, and wherein between 30% and 80% of itsexchangeable cations are sodium cations.
 11. The process of claim 1,wherein said adsorbent has been washed with an acid or alkali solutionprior to being placed in said bed.
 12. The process of claim 1, whereinsaid Li-exchanged zeolite is LiX.
 13. The process of claim 1, whereinsaid product gas contains less than 100 ppb of N₂O.
 14. The process ofclaim 1, wherein said product gas contains less than 50 ppb of N₂O. 15.The process of claim 1, wherein said product gas contains less than 10ppm of N₂O.
 16. The process of claim 8, wherein in said pressure swingadsorption process, adsorption takes place at a pressure between 1.0 to25 bar, and desorption takes place at a pressure between 0.2 to 10.0bar.
 17. The process of claim 8, wherein in said temperature swingadsorption process, adsorption takes place at a temperature between −70degrees Celsius and 80 degrees Celsius, and desorption takes place at agreater temperature than said adsorption.
 18. A process for theseparation of N₂O from a gas stream containing at least N₂O andnitrogen, said process comprising passing said gas stream over a bed ofadsorbent having a ΔN₂O working capacity of greater than or equal to3.56×10⁻⁴ at IBL.
 19. The process of claim 18, wherein said gas streamis air.
 20. The process of claim 18, wherein said adsorbent is selectedfrom the group consisting of clinoptilolite, chabazite and Li-exchangedzeoliteor combinations thereof.
 21. The process of claim 18, whereinsaid gas stream contains less than 100 ppm N₂O.
 22. The process of claim18, wherein the ΔN₂O/ΔN₂ selectivity is greater than or equal to2.23×10⁻⁴ at IBL.
 23. An adsorption apparatus for the removal of N₂Ofrom a gas containing N₂O, nitrogen and other components, said apparatuscomprising one (30) or more beds of at least a first adsorbent, whereinsaid first adsorbent is an N₂O selective adsorbent selected from thegroup consisting of clinoptilolite, chabazite and Li-exchanged zeoliteor combinations thereof.
 24. The apparatus of claim 23, wherein saidother components include H₂O and CO₂, and said apparatus furthercontains one or more additional adsorbents for the adsorption of H₂O andCO₂, and wherein said additional adsorbents are upstream of said firstadsorbent.
 25. The apparatus of claim 24, wherein said additionaladsorbents are one or more of cation exchanged natural zeolites, cationexchanged synthetic zeolites, alumina, silica gel and activated carbon.26. The apparatus of claim 24, wherein said N₂O selective adsorbent isin a separate layer from said additional adsorbents.
 27. The apparatusof claim 23, wherein said apparatus comprises a layer of alumina and amixed layer of said N₂O selective adsorbent and an adsorbent selectivefor CO₂ downstream of said layer of alumina.
 28. The apparatus of claim23, wherein said apparatus comprises a layer of alumina and, downstreamtherefrom, a layer of a composite material comprising said N₂O selectiveadsorbent and an adsorbent selective for CO₂ bound into a singleparticulate material.
 29. The apparatus of claim 23, wherein saidadditional adsorbents are in separate layers.
 30. The apparatus of claim23, wherein said adsorbent is in the form of particles having an averagesize that is selected between 4-50 mesh.
 31. The apparatus of claim 23,wherein said apparatus contains two beds (16, 17).
 32. The apparatus ofclaim 1, wherein said one or more beds contain a second additionaladsorbent for the removal of hydrocarbons.
 33. The apparatus of claim32, wherein said second additional adsorbent is either in its own layer,or is mixed with said first adsorbent(s).
 34. The apparatus of claim 23,wherein the combination of said first adsorbents is either in the formof a mixture of adsorbents or a composite adsorbent.
 35. The apparatusof claim 23, wherein said first adsorbents are clinoptilolite andchabazite, and wherein said clinoptilolite and chabazite are eithernatural or synthetic and have exchangeable cations selected from thegroup consisting of cations of Group 1A, Group 2A, Group 3A, Group 3B,the lanthanide series and combinations thereof.
 36. The apparatus ofclaim 23, wherein said apparatus is an air prepurifier.